Wednesday, July 18, 2018

Magnitude processing

Most modern NMR experiments are run in a phase sensitive mode, which means that after fourier transformation the peaks require phase correction to become all positive. The HMBC experiment, however, generates signals that cannot all be phased simultaneously. The commonly used solution is magnitude processing.

Wednesday, June 6, 2018

Linewidth titrations

NMR is one of the most powerful techniques to monitor molecular interactions because of the wealth of information it provides. The previous post described monitoring chemical shift changes during titrations, but chemical shifts are not the only NMR parameter that can change during titrations. Often linewidths change as well. By measuring linewidth changes the dissociation rate, koff, may be determined.

Tuesday, May 15, 2018

Chemical shift titrations

NMR is one of the most useful techniques for monitoring molecular interactions. It is particularly useful for characterizing weakly interacting systems. Typically, weakly interacting systems produce spectra that are in fast exchange, i.e. the NMR observables are detected as population weighted averages. By manipulating the populations, through changing concentration for example, a series of spectra can be obtained from which information like the dissociation constant (KD) and the dissociation rate (koff) may be determined. Titrating one molecule with another and monitoring the chemical shift changes is a common method for determining KD.

Thursday, April 5, 2018

Dynamic NMR

NMR is one of the few techniques that can provide dynamic information at the atomic level. It is able to do this because NMR parameters, like chemical shift, coupling constants and relaxation rates, are influenced by the immediate atomic environment. If the environment changes, then these NMR parameters are likely to change as well. The rate at which the environment changes affects how the NMR parameters appear in a spectrum. If the change is slower than the time taken to record that parameter then it is called "slow exchange", and multiple values of the NMR parameter are recorded. If the change happens faster than the time taken to record it then it is referred to as "fast exchange", and an averaged NMR parameter is detected. As an example, the temperature dependent chemical shifts of the thyroid hormone thyroxine are discussed below.

Friday, February 2, 2018

Homodecoupling

Homodecoupling is a technique for selectively removing coupling in 1D NMR spectra. Often 1D 1H spectra show complex splitting patterns that are difficult to interpret. Selectively eliminating one coupling may allow a multiplet to be interpreted or enable measurement of couplings that could not otherwise be quantified. Recently, we used homodecoupling to assign the stereochemistry of a pair of enantiomers.

Wednesday, January 10, 2018

Gradients for measuring diffusion

Translational diffusion, also known as Brownian motion, is the random motion of particles among the other components of a solution. The rate of diffusion is related to the size of the molecule so measuring this rate can provide an estimate of molecular weight and information on aggregation, or binding, at close to physiological conditions. NMR experiments have been devised to measure translational diffusion using a pair of gradient pulses separated by a delay. These Diffusion Ordered SpectroscopY, or DOSY1, experiments, are typically shown as 2D spectra with a horizontal 1H dimension and a vertical diffusion rate dimension.

Thursday, December 7, 2017

Gradients for coherence selection

In addition to artifact suppression and removing solvent peaks, gradient pulses can be used for coherence selection. Traditionally, this was done by phase cycling but using gradients for coherence selection allows cleaner spectra to be obtained more quickly. In this post the use of gradients to select coherences in heteronuclear experiments is discussed.